Luma mapping—and chroma scaling-based video or image coding

Abstract

According to the disclosure of the present document, for LMCS, a linear reshaper may be used, LMCS codewords (or ranges thereof) may be restricted, and in the chroma scaling of the LMCS, a single chroma residual scaling factor may be used. Accordingly, the resources/cost (of software or hardware) necessary for an LMCS procedure may be minimized, and latency in coding may be eliminated, thus enabling the LMCS procedure to be efficiently performed.

What is claimed is: 
     
       1. An image decoding method performed by a decoding apparatus, the method comprising:
 obtaining image information including luma mapping with chroma scaling (LMCS) related information from a bitstream; 
 deriving at least one LMCS codeword for an LMCS process based on the LMCS related information; 
 generating reconstructed luma samples based on the at least one LMCS codeword; and 
 generating modified reconstructed luma samples based on the reconstructed luma samples and the at least one LMCS codeword, 
 wherein the LMCS related information includes information on linear mapping, 
 wherein the at least one LMCS codeword is derived based on signaling of the information on the linear mapping, and 
 wherein the at least one LMCS codeword is a multiple of two. 
 
     
     
       2. The method of  claim 1 , wherein an LMCS delta codeword is derived based on the information on the linear mapping,
 wherein the at least one LMCS codeword is derived based on a sum of the LMCS delta codeword and OrgCW, 
 wherein OrgCW is equal to (1<<BitDepthY)/16, and 
 wherein BitDepthY represents a luma bit depth. 
 
     
     
       3. The method of  claim 1 , wherein an LMCS delta codeword is derived based on the information on the linear mapping,
 wherein the at least one LMCS codeword is derived based on a sum of the LMCS delta codeword and OrgCW* (lmcs_max_bin_idx-lmcs_min_bin_idx+1), 
 wherein lmcs_max_bin_idx and lmcs_min_bin_idx are a maximum bin index and a minimum bin index, respectively, 
 wherein OrgCW is (1<<BitDepthY)/16, and 
 wherein BitDepthY represents a luma bit depth. 
 
     
     
       4. The method of  claim 1 , wherein information on two pivot points is derived based on the information on the linear mapping,
 wherein the two pivot points include a first pivot point and a second pivot point, 
 wherein an x value and a y value of the first pivot point are a minimum input value and a minimum mapping value, respectively, 
 wherein an x value and a y value of the second pivot point are a maximum input value and a maximum mapping value, respectively, 
 wherein input values between the minimum input value and the maximum input value are linearly mapped. 
 
     
     
       5. The method of  claim 4 , wherein an x delta value of the first pivot point and a y delta value of the second pivot point are signaled based on the information on the linear mapping,
 wherein the maximum input value is derived based on the x delta value of the second pivot point, and 
 wherein the maximum mapping value is derived based on the y delta value of the second pivot point. 
 
     
     
       6. The method of  claim 5 , wherein the maximum mapping value is derived based on a following equation,
   lmcsCWAll=lmcs_ p 2_ y =(1<<bitdepth Y )−1−lmcs_ p 2_ y _delta  [Equation 16]
 
 wherein, lmcsCWALL and lmcs_p2_y represent the maximum mapping value, bitdepthY represents a luma bit depth, and lmcs_p2_y delta represents the y delta value of the second pivot point. 
 
     
     
       7. The method of  claim 1 , wherein the method further comprises deriving reconstructed chroma samples based on the LMCS related information, and
 wherein the reconstructed chroma samples are derived based on a single chroma residual scaling factor. 
 
     
     
       8. The method of  claim 7 , wherein the single chroma residual scaling factor is derived based on an average of inverse mapping gradients for deriving the modified reconstructed luma samples. 
     
     
       9. The method of  claim 7 , wherein based on signaling of the information on the linear mapping, a gradient reciprocal of the linear mapping is used as the single chroma residual scaling factor. 
     
     
       10. The method of  claim 7 , wherein parameters for the single chroma residual scaling factor are signaled based on the LMCS related information, and
 wherein the single chroma residual scaling factor is derived based on parameters for the single chroma residual scaling factor. 
 
     
     
       11. The method of  claim 1 , when a luma bit depth of the reconstructed luma samples is higher than  10 , the at least one LMCS codeword is a multiple of 1<<(BitDepthY-10), and
 wherein, BitDepthY represents the luma bit depth. 
 
     
     
       12. The method of  claim 1 , wherein the at least one LMCS codeword is in a range from (OrgCW>>1) to (OrgCW<<1)−1. 
     
     
       13. An image encoding method performed by an encoding apparatus, the method comprising:
 deriving at least one luma mapping with chroma scaling (LMCS) codeword; 
 generating residual information based on prediction for a current block; 
 generating reconstructed luma samples based on the at least one LMCS codeword and the residual information; 
 generating modified reconstructed luma samples based on the at least one LMCS codeword and the reconstructed luma samples; 
 generating LMCS related information based on the at least one LMCS codeword, the reconstructed luma samples, and the modified reconstructed luma samples; and 
 encoding image information including the residual information and the LMCS related information, 
 wherein the LMCS related information includes information on linear mapping, 
 wherein the at least one LMCS codeword is derived based on the information on the linear mapping, and 
 wherein the at least one LMCS codeword is a multiple of two. 
 
     
     
       14. A decoding apparatus for image decoding, the decoding apparatus comprising:
 a memory; and 
 at least one processor connected to the memory, the at least one processor configured to: 
 obtain image information including luma mapping with chroma scaling (LMCS) related information from a bitstream; 
 derive at least one LMCS codeword for an LMCS process based on the LMCS related information; 
 generate reconstructed luma samples based on the at least one LMCS codeword; and 
 generate modified reconstructed luma samples based on the reconstructed luma samples and the at least one LMCS codeword, 
 wherein the LMCS related information includes information on linear mapping, 
 wherein the at least one LMCS codeword is derived based on signaling of the information on the linear mapping, and 
 wherein the at least one LMCS codeword is a multiple of two.